How to get diffuse color of a material via Python?
Python language itself is only a small part of the whole experience. You should expect Blender's and Maya's Python APIs to be very different. 

The materials are assigned to material slots of objects in Blender, every object's faces have material indices matching those slots.

You can reach your selected objects using context. That would be bpy.context.selected_objects. You can loop through them.

Faces are stored in object's data, so, you would need to work with a specific object for example first one from the selection bpy.context.selected_objects[0].data.polygons. You can loop trough them and see material indices:

import bpy

for face in bpy.context.selected_objects[0].data.polygons: 
     print('Material index of face ' + str(face.index) + ' : ' + str(face.material_index))


You can change material indices, but you do not need to do that if you only want to assign a material to an object. You can just add a material to an object as described here that will create the slots automatically. You can see material slots bpy.context.selected_objects[0].material_slots and materials assigned to them bpy.context.selected_objects[0].material_slots[0].material.name 

You can also reassign materials to material slots. Materials are stored in bpy.data.materials so let's say you wanted to change an existing material that is assigned to the active object's already existing second material slot with one that is called 'Material':

import bpy

obj = bpy.context.active_object
material = bpy.data.materials.get("Material")
obj.data.material_slots[1].material = material


or you could just make a new slot:

obj.data.materials.append(material)


By this point I am sure you are frustrated about how in the world one is supposed to know where what is in Blender's Python API. Well there is a nice structure there that you can explore easily with the auto complete function of the Python Console in Blender. Just start typing and hit ctrl+space: 



You can see everything there. That is how I found that faces have material indices: 



For functions and operators you can just use Google, that will most likely point you to Blender Stack Exchange or Blender's Python API documentation that is really useful. 

You can also explore Text Editor's Templates menu for examples and also every single add-on that you get your hands on is a great resource for learning. 

Edit: Oh, I forgot to actually answer the question. You can get the diffuse color of your selected objects like this(assuming you use Cycles and you use nodes in the materials):

import bpy
C=bpy.context

for o in C.selected_objects:
    if o.type in {'MESH', 'SURFACE'} and len(o.material_slots) &gt; 0: 
        i = 0
        for s in o.material_slots:
            if s.name:
                print('Object ' + o.name + ' has a material "' 
                      + s.material.name + '" in slot ' + str(i)
                     )
                if s.material.use_nodes:
                    for n in s.material.node_tree.nodes:
                        if n.type=='BSDF_DIFFUSE':
                            print("The color of it's Diffuse BSDF named '" 
                                  + n.name + "' is " 
                                  + str(n.inputs[0].default_value[:]) 
                                  + '\n'
                                 )
            i += 1